Screen printing apparatus having paste chamber with discharge opening and structure for introducing paste residue from previous printing into discharge opening

ABSTRACT

A screen printing apparatus for printing on a board, such as a circuit board, through a screen using a paste prior to surface-mounting electronic components on the board, and more particularly a screen printing apparatus that performs a screen printing while automatically discharging the paste that is accommodated inside the main body.

TECHNICAL FIELD

The present invention relates to a screen printing apparatus forprinting on a board, such as a circuit board, through a screen usingpaste such as solder paste made by kneading a mixture of solder powderand liquid flux, prior to an operation of surface-mounting of electroniccomponents on the board, and more particularly to a screen printingapparatus capable of performing screen printing while automaticallydischarging, through an opening formed at the bottom of a housing-likeapparatus main body, the paste that is accommodated inside the mainbody.

BACKGROUND ART

A screen printing apparatus is known from JP-A-5 (1993)-57870 and U.S.Pat. No. 5,947,022 (Jul. 9, 1999), for example. In the screen printingapparatus of this type, a pair of scrapers or blades is provided insidethe apparatus main body so as to be inclined inward and opposed to eachother. Solder paste accommodated inside the main body is pushed out ofthe opening opened between the scrapers or blades with air pressure ordischarge pressure produced by rotation of a rotary body, and screenprinting is performed by fluid pressure of the paste discharged from thedischarge opening. A surplus of the paste is collected by scraping itwith one of the pair of scrapers or blades that follows the other duringscreen printing.

In the screen printing performed by using such a known type of screenprinting apparatus, it is unavoidable that the liquid flux includingsolder particles leaks out from between the scrapers or blades and ascreen during printing and remains on the screen for, for example, thefollowing reasons: a desired number of printing operations are repeatedautomatically; in general, screens have deformed portions and smallasperity; and solder particles and flux as components of the solderpaste tend to be separated from each other by shearing force generatedby the discharge pressure exerted on the solder paste during printing.Therefore, to obtain good printing results, it is necessary tofrequently clean and remove the paste residue adhered to the screenmanually or with a cleaning device. In this case, the paste residueremoved from the screen is disposed of as wastes. However, solder pasteis as expensive as 50,000 to 300,000 yen per 100 g and accordingly,disposal of reusable paste residue is uneconomical. Moreover, printingoperations need to be suspended during a cleaning operation, whichcauses the efficiency of the printing operations to be lowered.

The present invention has been made to eliminate the drawbacks ordisadvantages of the prior art as mentioned above.

It is, therefore, the main object of the present invention to provide ascreen printing apparatus in which in each printing, a residual of thepaste adhered to the upper surface of a screen in the preceding printingcan be introduced and used for printing.

Another object of the invention is to provide a screen printingapparatus in which a paste accommodation chamber is provided inside theapparatus main body and the paste accommodated in the accommodationchamber is stirred and kneaded efficiently and sufficiently anddischarged in the state in which its viscosity is adjusted so as to besuitable for printing.

A further object of the invention is to provide a screen printingapparatus which can be disassembled easily so as to be capable offacilitating cleaning and maintenance, inspection, or the like of maincomponents such as the main body and a paste stirring unit.

Other objects of the invention will become apparent from the descriptionwhich will be made in respect of its embodiments hereinafter.

DISCLOSURE OF THE INVENTION

To attain the above-mentioned main object of the present invention,there is provided in accordance with the invention a screen printingapparatus having a housing-like main body that is long sideways,characterized in that inside the main body, there are provided a pasteaccommodation chamber and a roller pivotally mounted along the axis ofthe main body that a discharge opening that is long sideways and servesto discharge the paste accommodated in the accommodation chamber ontothe upper surface of a screen placed on a board is formed at the bottomof the main body between a scraper that projects forward and diagonallydownward and whose tip slides on the screen upper surface duringprinting while being in contact therewith and a guide plate that isopposed to the scraper and projects backward and diagonally downward andwhose tip ends at such a height where a predetermined gap is maintainedfrom the tip to the screen upper surface during printing, so that as themain body is moved forward, that is, in a printing direction and theroller is rotated in one direction, the paste can be filled in patternholes of the screen by fluid pressure of the paste discharged from thedischarge opening, and that a front wall of the main body is formed witha space for introducing a residue of the paste adhered to the screenupper surface in the preceding printing to the discharge opening throughsaid gap in each printing so that the paste residue is mixed into thepaste discharged from the discharge opening and can be used forprinting.

To attain the main object of the present invention, there is provided inaccordance with the invention a screen printing apparatus having ahousing-like main body that is long sideways, characterized in thatinside the main body, there are provided a paste accommodation chamberand a roller pivotally mounted along the axis of the main body so as tobe capable of being rotated in normal and reverse directions, that adischarge opening is long sideways and serves to discharge the pasteaccommodated in the paste accommodation chamber onto the upper surfaceof a screen placed on a board is formed at the bottom of the main bodybetween a scraper that projects forward and diagonally downward andwhose tip slides on the screen upper surface during printing while beingin contact therewith and a guide plate that is opposed to the scraperand projects backward and diagonally downward and whose tip ends at aheight where a predetermined gap is maintained from the tip to thescreen upper surface during printing, so that as the main body is movedforward, that is, in a printing direction and the roller is rotated inthe normal direction, the paste can be filled in pattern holes of thescreen by fluid pressure of the paste discharged from the dischargeopening, that a front wall of the main body is formed with a space forintroducing a residue of the paste adhered to the screen upper surfacein the preceding printing to the discharge opening through said gap ineach printing, so that the paste residue is mixed into the pastedischarged from the discharge opening can be used for printing, and thatas the roller is rotated in the reverse direction at the end of aprinting operation, the paste remaining in the vicinity of the dischargeopening is introduced and collected into the paste accommodationchamber.

There is also provided in accordance with the invention a screenprinting apparatus, a scraper that is made of a thin steel strip havinga predetermined rigidity and elasticity and is fixed to a bottom portionof a rear wall of the main body by means of a fixing member, that thetip of the scraper is rounded and projects slightly downwardly of thebottom of the main body in a non-printing state, and that the guideplate is screwed to the a lower portion of the front wall of the mainbody.

Furthermore, there is provided in accordance with the invention a screenprinting apparatus, characterized in that the roller is disposed closeto the discharge opening so as to be in sliding contact with a lowprojection strip that projects from the scraper parallel with the axisof the roller, and forms, between said projection strip itself and aguide surface of the guide plate, a narrow guide passage that serves toincrease paste discharge pressure.

To attain the main object of the present invention, there is provided inaccordance with the invention a screen printing apparatus, characterizedin that a stirring unit for stirring the paste received in the pasteaccommodation chamber is mounted on the apparatus main body in adetachable manner, said stirring unit comprising an elongate stirringplate having a number of stirring members that are arranged in line atpredetermined intervals in a longitudinal direction, a reciprocatingmechanism for reciprocating the stirring plate along the axis of theroller, and a device frame that holds the reciprocating mechanism.

In addition, to attain the main object of the present invention, thereis provided a screen printing apparatus, characterized in that thestirring members are integrally formed with the stirring plate and eachof the stirring members has a dog-legged stirring portion that extendsdownward from the stirring plate and terminates in a guide passage.

To attain the main object of the present invention, there is provided inaccordance with the invention a screen printing apparatus, characterizedin that it is provided with a driving unit for driving the roller forrotation, the driving unit comprising a housing that is formedseparately from the apparatus main body, and that the roller isoperatively coupled to said driving unit via a projected shaft portionof the roller that penetrates through one side wall of the apparatusmain body and projects outward.

To further attain the main object of the present invention, there isprovided in accordance with the invention a screen printing apparatus,characterized in that it is provided with a paste supply device whichcomprises a syringe-like container detachable attached to a bracketprojecting outward from the front wall of the main body, an outletopening of the container being connected to the paste accommodationchamber via a connection pipe, and pressurizing means capable ofpneumatically transporting the paste contained in the container into thepaste accommodation chamber under pressure, when necessary.

To attain the main object of the present invention, there is provided inaccordance with the invention a screen printing apparatus, wherein saidpaste accommodation chamber contains a paste, said paste is solder pasteobtained by kneading a mixture of solder alloy powder and liquid flux.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a screen printing apparatus as oneembodiment of the present invention;

FIG. 2 is a vertical cross-sectional side view of the apparatus of FIG.1 in which hatching is omitted;

FIG. 3 is an enlarged vertical cross-sectional side view of part A′ inFIG. 2;

FIG. 4 is a front view of the apparatus of FIG. 1 with some of itsportions omitted;

FIG. 5 is a vertical cross-sectional side view of a screen printingapparatus as another embodiment of the invention;

FIG. 6 is an enlarged vertical cross-sectional view of part A′ in FIG.5;

FIG. 7 is a partially cutaway perspective view of the apparatus of FIG.5;

FIG. 8 is a perspective view of the same apparatus that is orientedupside down;

FIG. 9 is an exploded perspective view of the same apparatus;

FIG. 10 is a perspective view of a stirring unit that is oriented upsidedown;

FIG. 11 illustrates how the apparatus operates; and

FIG. 12 illustrates how the apparatus operates immediately after the endof a printing.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be hereinafter described in detail withreference to the accompanying drawings that illustrate embodiments ofthe invention.

In the drawings in FIGS. 1–4, reference character A denotes ahousing-like main body, long sideways, of a screen printing apparatus.In the main body A, there are provided an accommodation chamber 1 foraccommodating solder paste H and a roller 2 pivotally mounted along theaxis of the main body. The bottom portion of the main body A has thefollowing structure. A scraper 3 projects forward and diagonallydownward and, during printing, whose tip slides on the upper surface ofa screen S, which is placed on a board P, while being in contact withsaid surface. A guide plate 5 extends backward and diagonally downwardso as to be opposed to the scraper 3 and, during printing, whose tipterminates at a height where a predetermined gap 4 is maintained fromthe tip to the upper surface of the screen S. A discharge opening 6,long sideways, is formed between the scraper 3 and the guide plate 5. Asthe main body A is moved forward, that is, in the printing directionindicated by arrow X and the roller 2 is rotated in one direction, thatis, clockwise in FIGS. 2 and 3, the solder paste H accommodated in theaccommodation chamber 1 is discharged through the discharge opening 6and filled in pattern holes S′ of the screen by fluid pressure of thepaste discharged from the discharge opening 6 and, at the same time, thescraper 3 scrapes up a surplus of the solder paste from the screen. Aspace 7 that forms a tunnel-like gap between a front wall 9, whichextends between side walls 8 of main body A, and the upper surface ofscreen S is provided on the underside of the front wall 9. This space 7serves, in each printing operation, to introduce solder paste residuesH′ (see FIGS. 11 and 12) that were adhered to the upper surface of thescreen S in the preceding printing operation and to guide them to thedischarge opening 6 through the aforementioned gap 4. Therefore, as themain body A advances, the paste residues H′ are introduced from thespace 7 to discharge opening 6 through the gap 4 and mixed into thesolder paste H discharged from the discharge opening 6 so as to becapable of being used for printing.

In the illustrated embodiment, the roller 2 is mounted pivotally insidethe main body A so as to be able to rotate in opposite directions, i.e.,the normal and reverse directions and to be accelerated and decelerated.By rotating the roller 2 clockwise, i.e., in the normal direction, thepattern holes S′ of screen S can be charged with solder paste H by fluidpressure of the paste H discharged from the discharge opening 6.Further, by rotating the roller 2 in the reverse direction after the endof a printing operation, solder paste H remaining close to the dischargeopening 6 can be collected by introducing it into the pasteaccommodating chamber 1.

In the illustrated embodiment, the scraper 3 is made of a thin stripsteel, such as a spring steel having rigidity and elasticity and isfixed to a bottom portion of a rear wall 10 of the main body A by meansof a fixing member 11. The tip of the scraper 3 is rounded to make itsfriction coefficient small so as to prevent scratches from being formedon the upper surface of screen S and projects slightly downward from thebottom surface of the main body A when the apparatus is not inoperation. The guide plate 5 is screwed to a lower inside portion of thefront wall 9 of main body A. The roller 2 is disposed close to thedischarge opening 6 in such a manner as to be in sliding contact with astrip 12 that projects from the scraper 3 parallel with the axis of theroller 2, and forms, between itself and a guide surface 5′ of the guideplate 5, a narrowed paste guide passage 13 that serves to increase thedischarge pressure of solder paste H.

Reference numeral 14 denotes a steel partition plate which is screwed toan inside surface of the rear wall 10 at its upper portion in such amanner as to project horizontally from the inside surface and that has,at its forward end, a guide member 15 having an arc-shapedcross-section. The lower end of the guide member 15 is curved forward toform a concave and is elastically brought in contact with thecircumferential surface of the roller 2 over its entire longitudinallength. Therefore, the solder paste H in the paste-accommodating chamber1 is prevented from going around the roller 2 to reach its back side.

Reference numeral 16 denotes a leak-prevention projection that projectsslightly inward from an inside lower portion of each side wall 8 andthat has a function of preventing leakage of the solder paste H from alower portion of the end surface of each of the scrapers 3 and guideplate 5 by being brought in pressure contact with the lower portion ofthe end surface. A piece of sheet 17 that is made from engineeringplastics (e.g., PTFE) having superior chemical resistance and a smallfriction coefficient and that is brought in sliding contact with theupper surface of screen plate S is fixedly secured to the bottom surfaceof each of the projections 16. This structure allows a smooth slide ofmain body A on the screen S and prevents early wear of the screen.Reference numeral 18 denotes a shaft coupling fixed to a projected shaftportion of the roller 2 that penetrates through one side wall 8 andprojects outward. The shaft coupling 18 is connected to a roller drivingunit which will be described later and which houses a motive powersource for driving the roller for rotation.

FIGS. 5–10 show another embodiment of the present invention in which theaforementioned body A is combined with a paste stirring device B forstirring solder paste H in the accommodation chamber 1. The pastestirring device (hereinafter referred to as “stirring unit”) B iscomposed of an elongate stirring plate 20 having a large number ofstirring members 19 that are arranged in line at predetermined intervalsin the longitudinal direction, a reciprocating mechanism 21 forreciprocating the stirring plate 20 along the roller 2, and a deviceframe 22 that holds the reciprocation mechanism 21. Thus, the pastestirring device B is formed as a unit that is separate from the mainbody A and is detachable attached thereto. Reference character Dindicates a paste supply device for supplying paste into theaccommodation chamber 1. This supply device D includes a syringe-likecontainer D′ which can be detachable fixed to a bracket 9A that projectshorizontally from outside of the front wall 9 of main body A. An outletopening D-1 of the container D′ is connected to a lower portion of thepaste accommodation chamber 1 via a connection passage D-2. The pastesupply device D is provided with a pressurizing means (not shown) forsupplying the paste contained in the container into the pasteaccommodation chamber 1 under pressure, when necessary. In theillustrated example, the pressurizing means consists of a pneumaticsystem. In FIGS. 5–7, D-3 denotes an air pressure introduction portionprovided in a lid of the container D′.

In the illustrated example, as clearly shown in FIGS. 6, 7, 9, and 10,the stirring members 19 are integrally formed with the stirring plate 20and each stirring member 19 consists of a dog-legged stirring piece 23that extends downward from the stirring plate 20 and a stirringprojection 24 that projects approximately horizontally from a middleportion of the stirring piece 23 in the direction in which the stirringpiece is bent. In a state that the stirring unit B is attached to theapparatus main body A, as clearly shown in FIGS. 5–7, the dog-leggedstirring pieces 23 reach in the guide passage 13. Portions 23′ on thetip side of the bending portions are located close to the portion of thecircumferential surface of the roller that is below the horizontal planepassing through the center of the roller 2. The stirring projections 24extend right above the roller 2 and their round tips 24′ face to aconcave surface 15′ of the guide 15 having the arc-shaped cross section.

Referring to FIG. 9, reference character C denotes the aforementionedroller driving unit (hereinafter referred to as “driving unit”) that isa unit which is separate from either of the main body A and the stirringunit B. The driving unit C is composed of a vertically elongate housing25 that houses a motor as a motive power source for driving the roller 2for rotation and a transmission mechanism therefor (both not shown) andan engagement plate 26 that projects horizontally from the housing andserves to attach the housing 25 to the stirring unit B. The engagementplate 26 of the housing 25 is engaged with an engagement groove 27 of atop plate 22A of the device frame 22 and fixed to it with set screws 28in a state that the stirring unit B is attached to the main body A,whereby the driving unit C is fixed to the stirring unit B in the statein which the aforementioned shaft coupling 18 of roller 2 is operativelycoupled to the motor. In the illustrated example, the stirring unit Bcan easily be attached to or detached from the main body A by hooking orunhooking an engagement member 30 provided at the end of the deviceframe 22 of stirring unit B on or from a locking piece 29 that projectsoutward from the side wall 8 of the main body A. The above structuremakes it possible to easily perform cleaning, maintenance and inspectionof each of the main body A, the stirring unit B, and the driving unit Cby separating them from each other.

FIG. 11 illustrates the state in which the main body A to which thestirring unit B is attached is performing a printing operation. Solderpaste residues H′ that stuck to the screen S in the preceding printingoperation remain on the screen placed on the board P. As the main body Ais made to advance while the roller 2 is rotated clockwise after themain body A was set at a printing start position on the screen andoperation of the stirring unit B was started, the solder paste Haccommodated in the accommodation chamber 1 is stirred and kneadedefficiently and sufficiently with the rotation of roller 2 and thereciprocation of stirring members 19 of the stirring unit B andviscosity of the paste is adjusted so as to be suitable for printing.Solder paste H is discharged through the discharge opening 6 in thestate in which it has been pressurized hydrodynamically in the narrowpaste guide passage 13 by rotation of the roller 2, and is forced to befilled in a plurality of pattern holes S′ of the screen S in turn by itsfluid pressure. At the same time, the solder paste residues H′ remainingon the screen S are introduced into the tunnel-like space 7, guided tothe discharge opening 6 through the gap 4 between the screen S and theguide plate 5, mixed into the solder paste H discharged from thedischarge opening 6 while being kneaded, and finally forced to be filledin the pattern holes S′ of the screen together with the paste. In thiscase, in general, paste residues H′ contain more flux than solderparticles and hence do not adversely affect the printing effects.Conversely, paste residues H′ supply flux to the paste and hence bringabout favorable effects.

FIG. 12 illustrates the manner in which the apparatus is operated at orafter the end of a printing operation. As the apparatus main body Aterminates the printing at a printing end position on the screen S andthe roller 2 is rotated counterclockwise, i.e., in the reversedirection, solder paste H remaining close to the discharge opening 6 iscollected, that is, introduced into the accommodation chamber 1. In thiscase, as the roller 2 is continuously rotated in the reverse direction,paste H is caused to be adhered to the peripheral surface of the roller2 and to be transported in the reverse direction by the rotating roller.Then, as shown in FIG. 12, the paste adhered to said surface is scrapedoff the roller 2 by the guide member 15 that is in sliding contact withthe roller surface, and is rolled while being guided by the concaveguide surface 15′ of guide member 15. Therefore, the paste does not goaround the guide member to reach its back side. To collect the pastequickly, it is desirable to increase the speed of the reverse rotationof the roller 2. After the paste has been collected in the above manner,no paste or only a very small amount of paste remains close to thedischarge opening 6. Therefore, it is made possible to omit an operationof cleaning the upper surface of the screen S.

INDUSTRIAL APPLICABILITY

Thus, the screen printing apparatus constructed according to theinvention can solve the aforementioned problems of the prior artreferred to above. Further, in each of repeated printing operations, thescreen printing apparatus makes it possible to take therein undriedpaste residues that stuck to the upper surface of the screen in thepreceding printing operation and remain thereon and to utilize them forprinting. This provides many advantages, for example, it greatlyincreases the efficiency of screen printing operations, reduces thecost, and enables a continuous operation for a long period of time.

1. A screen printing apparatus having a housing-like main body that is long sideways, characterized in that inside the main body, there are provided a paste accommodation chamber and a roller pivotally mounted along the axis of the main body, that a discharge opening that is long sideways and serves to discharge a paste accommodated in the paste accommodation chamber onto the upper surface of a screen placed on a printed circuit board is formed at the bottom of the main body between a scraper that projects forward and diagonally downward and whose tip slides on the screen upper surface during printing while being in contact therewith and a guide plate that is opposed to the scraper and projects backward and diagonally downward and whose tip ends at a height where a predetermined gap is maintained from the tip to the screen upper surface during printing, so that as the main body is moved forward, that is, in a printing direction and the roller is rotated in one direction, the paste can be filled in pattern holes of the screen by fluid pressure of the paste discharged from the discharge opening, and that a front wall of the main body is formed with a space for introducing a residue of the paste adhered to the screen upper surface in the preceding printing to the discharge opening through said gap in each printing, so that the paste residue is mixed into paste discharged from the discharge opening and can be used for printing.
 2. The screen printing apparatus according to claim 1, characterized in that the scraper is made of a thin steel strip having a predetermined rigidity and elasticity and is fixed to a bottom portion of a rear wall of the main body by means of a fixing member, that the tip of the scraper is rounded and projects slightly downwardly of the bottom of the main body in a non-printing state, and that the guide plate is screwed to a lower portion of the front wall of the main body.
 3. The screen printing apparatus according to claim 1, characterized in that the roller is disposed close to the discharge opening so as to be in sliding contact with a low projection strip that projects from the scraper parallel with the axis of the roller, and forms, between said projection strip and a guide surface of the guide plate, a narrow guide passage that serves to increase paste discharge pressure.
 4. The screen printing apparatus according to claim 1, characterized in that a stirring unit for stirring the paste received in the paste accommodation chamber is mounted on the apparatus main body in a detachable manner, said stirring unit comprising an elongate stirring plate having a number of stirring members that are arranged in line at predetermined intervals in a longitudinal direction, a reciprocating mechanism for reciprocating the stirring plate along the axis of the roller, and a device frame that holds the reciprocating mechanism.
 5. The screen printing apparatus according to claim 4, characterized in that the stirring members are integrally formed with the stirring plate and each of the stirring members has a dog-legged stirring portion that extends downward from the stirring plate and terminates in a guide passage.
 6. The screen printing apparatus according to claim 1, characterized in that it is provided with a driving unit for driving the roller for rotation, the driving unit comprising a housing that is formed separately from the apparatus main body, and that the roller is operatively coupled to said driving unit via a projected shaft portion of the roller that penetrates through one side wall of the apparatus main body and projects outward.
 7. The screen printing apparatus according to claim 1, characterized in that it is provided with a paste supply device which comprises a syringe-like container detachably attached to a bracket projecting outward from the front wall of the main body, an outlet opening of the container being connected to the paste accommodation chamber via a connection pipe, and pressurizing means capable of pneumatically transporting the paste contained in the container into the paste accommodation chamber under pressure, when necessary.
 8. The screen printing apparatus according to claim 1, wherein said paste accommodation chamber contains a paste, said paste is solder paste obtained by kneading a mixture of solder alloy powder and liquid flux.
 9. A screen printing apparatus having a housing-like main body that is long sideways, characterized in that inside the main body, there are provided a paste accommodation chamber and a roller pivotally mounted along the axis of the main body so as to be capable of being rotated in normal and reverse directions, that a discharge opening that is long sideways and serves to discharge a paste accommodated in the paste accommodation chamber onto the upper surface of a screen placed on a printed circuit board is formed at the bottom of the main body between a scraper that projects forward and diagonally downward and whose tip slides on the screen upper surface during printing while being in contact therewith and a guide plate that is opposed to the scraper and projects backward and diagonally downward and whose tip ends at a height where a predetermined gap is maintained from the tip to the screen upper surface during printing, so that as the main body is moved forward, that is, in a printing direction and the roller is rotated in the normal direction, the paste can be filled in pattern holes of the screen by fluid pressure of the paste discharged from the discharge opening, that a front wall of the main body is formed with a space for introducing a residue of the paste adhered to the screen upper surface in the preceding printing to the discharge opening through said gap in each printing, so that the paste residue is mixed into the paste discharged from the discharge opening can be used for printing, and that as the roller is rotated in the reverse direction at the end of a printing operation, the paste remaining in the vicinity of the discharge opening is introduced and collected into the paste accommodation chamber. 